Fleas are light brown, dark brown and reddish brown insects whose ugly appearance belies their wingless, blood-sucking functionality. Ranging in size from less than one-tenth of an inch to up to three-tenths of an inch, a flea's two most prominent physical features are a laterally flattened body build, to be able to nimbly navigate between the hairs of a host body, and the long claws affixing the end of each of its six legs. 

Incredible Jumping Power 

Since these and other features are only properly seen through the lens of a magnifying glass or microscope, the physiological aspect of fleas that humans are most familiar with is the incredible jumping power of this insect's hind legs. Given the fact that a flea can propel itself through the air at a distance of up to 150 times its body length, the scientific term for the species - Siphonaptera, or "wingless siphon" - is somewhat moot. In essence, a flea really can fly through the air when it needs to escape capture or jump onto a host body. 

Flea Life Cycle 

Fleas begin life as tiny, light-colored worm-like larvae. A female flea can lay up to 50 eggs per day and live as long as 25 days, resulting in potential payload of 1,250 larvae hatching in all corners of an infested area. The life cycle of a flea progressing from the egg stage to adulthood can take up to a month, allowing for the detection of tiny spindle cocoons on an infested dog or cat's body.

Fierce Characteristics 

A number of flea characteristics reflect its fierce, pre-ordained determination not to be removed from a host's body. For example, the spines that coat a flea's mouth, back and legs are there to make it that much harder for a flea to be loosened and removed when a pet is groomed or brushed. Along with cat fleas ("Ctenocephalides felis"), dog fleas("Ctenocephalides canis") and human fleas ("Pulex irritans"), there is also a rarer form of oriental and northern rat flea ("Xenopsylla cheopis", "Nosopsyllus fasciatus")  that can be brought into households by Norway and roof rats.